The present invention relates to a control system for converters (inverter/rectifier) of a DC power transmission line.
FIG. 1 shows a prior art converter control system for a DC power transmission line. Such a control system is generally known from the following publication:
International Conference on Large High Voltage Electric Systems PA1 1980 session (August 27-September 4) tilted: "HOKKAIDO-HONSHU HVDC LINK" by T. Takenouch et al. (Japan)
Details of each element shown in FIG. 1 are known to a skilled person in the art. In FIG. 1, the DC circuit of a converter 1A is coupled via a DC reactor 2A, DC power transmission lines 3 and a DC reactor 2B to the DC circuit of a converter 1B. The AC circuit of converter 1A is coupled via a converter transformer 4A and a circuit breaker 5A to a 3-phase AC power line 6A, and the AC circuit of converter 1B is coupled via a converter transformer 4B and a circuit breaker 5B to a 3-phase AC power line 6B. Converter 1A is associated with an automatic voltage regulator 11A, an automatic extinction angle regulator 12A and an automatic current regulator 13A. Converter 1B is associated with an automatic voltage regulator 11B, an automatic extinction angle regulator 12B and an automatic current regulator 13B. Automatic extinction angle regulator 12A is provided for a prescribed operation that the extinction angle of converter 1A traces or follows a reference extinction angle value E18A obtained from a extinction angle presetter 18A, and automatic extinction angle regulator 12B is provided for a prescribed operation that the extinction angle of converter 1B follows a reference extinction angle value E18B obtained from an extinction angle presetter 18B.
The value of a detected DC voltage obtained from a DC voltage detector 15A is properly changed via a voltage/voltage converter 16A to a value being well adapted to the control circuitry of FIG. 1. The changed value obtained from voltage/voltage converter 16A is used as a DC voltage detected value E16A. DC voltage detected value E16A is subtracted in a summing circuit 17A from a reference voltage value E14A obtained from a DC voltage presetter 14A. Summing circuit 17A supplies to automatic voltage regulator 11A the difference between DC voltage detected value E16A and reference voltage value E14A, so that the DC voltage of DC power transmission line 3 at converter 1A side follows the reference voltage value E14A from DC voltage presetter 14A.
The value of a detected DC voltage obtained from a DC voltage detector 15B is properly changed via a voltage/voltage converter 16B to a value being well adapted to the control circuitry of FIG. 1. The changed value obtained from voltage/voltage converter 16B is used as a DC voltage detected value E16B. DC voltage detected value E16B is subtracted in a summing circuit 17B from a reference voltage value E14B obtained from a DC voltage presetter 14B. Summing circuit 17B supplies to automatic voltage regulator 11B the difference between DC voltage detected value E16B and reference voltage value E14B, so that the DC voltage of DC power transmission line 3 at converter 1B side follows the reference voltage value E14B from DC voltage presetter 14B.
The detected DC current value obtained from a DC current detector 21A is converted via a current/voltage converter 22A to a value being well adapted to the control circuitry. The converted value obtained from current/voltage converter 22A is used as a DC current detected value E22A. DC current detected value E22A is subtracted in a summing circuit 23A from a reference current value E26A obtained from a transmission control circuit 26A. Summing circuit 23A supplies to automatic current regulator 13A the difference between DC current detected value E22A and reference current value E26A, so that the DC current of DC power transmission line 3 at converter 1A side follows the reference current value E26A from transmission control circuit 26A.
The detected DC current value obtained from a DC current detector 21B is converted via a current/voltage converter 22B to a value being well adapted to the control circuitry. The converted value obtained from current/voltage converter 22B is used as a DC current detected value E22B. DC current detected value E22B is subtracted in a summing circuit 23B from a reference current value E26B obtained from a transmission control circuit 26B. Summing circuit 23B supplies to automatic current regulator 13B the difference between DC current detected value E22B and reference current value E26B, so that the DC current of DC power transmission line 3 at converter 1B side follows the reference current value E26B from transmission control circuit 26B.
A current margin value E25A or E25B obtained from a current margin presetter 25A or 25B is supplied via the closed one of switches 24A and 24B to the corresponding summing circuit 23A or 23B. Of switches 24A and 24B, only the one which allows the corresponding converter to operate as an inverter is closed. An advanced control angle preference circuit 28A receives outputs from regulators 11A, 12A and 13A. Advanced control angle preference circuit 28A selects only one of the received outputs in a manner that the selected one has the most phase-advanced control angle. An advanced control angle preference circuit 28B receives outputs from regulators 11B, 12B and 13B, and selects only one having the most phase-advanced control angle.
It is now assumed that switch 24B is closed while switch 24A is opened. In this case, according to the function of said current margin value and to the selecting function of advanced control angle preference circuits 28A and 28B, circuit 28A selects the output from automatic current regulator 13A and circuit 28B selects one having the most phase-advanced control angle of outputs from automatic voltage regulator 11B and automatic extinction angle regulator 12B. Circuit 28B generally selects the output from automatic voltage regulator 11B.
The selected output from circuit 28A is supplied to a phase control circuit 29A in which the selected output is converted into pulse signals which determine the triggering timing of the corresponding converter 1A. The selected output from circuit 28B is supplied to a phase control circuit 29B in which the selected output is converted into pulse signals which determine the triggering timing of the corresponding converter 1B. The pulse signals from phase control circuit 29A are supplied as gate pulses to converter 1A via a pulse amplifier 30A. The pulse signals from phase control circuit 29B are supplied as gate pulses to converter 1B via a pulse amplifier 30B.
It is conventional to arrange a converter control system in the manner described above. It is also known from, e.g., Japanese Patent Publication (Kokoku) No. 46-33255 that a typical operation characteristic curve of DC power transmission utilized in such a converter control system becomes as shown in FIG. 2. In FIG. 2, a DC current Id is plotted along the axis of abscissa and a DC voltage Ed is plotted along the axis of ordinate. Since it has been assumed that switch 24A is opened and switch 24B is closed, converter 1A serves as a rectifier (forward converter) while converter 1B serves as an inverter (reverse converter). The following description will be made under this assumption.
A curve having portions (a), (b) and (c) in FIG. 2 represents the characteristic of converter 1A which operates as a rectifier. A curve having portions (d), (e), (f) and (g) shows the characteristic of converter 1B which operates as an inverter. The intersecting point of the curve of (a) to (c) and the curve of (d) to (g) defines the operational point of the DC power transmission line. Of portions (a), (b) and (c), portions (a) and (b) represent the regulation curve which depends mainly on the commutating impedance of converter 1A. Portions (b) and (c) represents a constant current characteristic curve obtained by the actuation of automatic current regulator 13A. Of portions (d), (e), (f) and (g), portions (d) and (e) represents a constant current characteristic curve obtained by the actuation of automatic current regulator 13B. Portions (e) and (f) represents a constant voltage characteristic curve obtained by the actuation of automatic voltage regulator 11B. Portions (f) and (g) represents a constant extinction angle characteristic obtained by the actuation of automatic extinction angle regulator 12B. The difference between a DC current IdA given by portions (b, c) and a DC current IdB given by portions (d, e) corresponds to the current margin which is generally set to be 5% to 10% of the rated DC current (IdA). For instance, if the rated current IdA is set at 10.0 A, the DC current IdB is 9.0 A to 9.5 A.
The reason why the current margin is set at 5% to 10% of the rated DC current will now be described.
It is assumed that the current margin is set at a very large value so that no constant current characteristic appears at converter 1B (i.e., IdB=0). In this case, the operation characteristic curve of converter 1B may be represented by portions (h), (f) and (g) in FIG. 3. Referring to FIG. 3, portions (h) and (f) represent the constant voltage characteristic obtained by the actuation of automatic voltage regulator 11B. Portions (f) and (g) represent the constant extinction angle characteristic obtained by the actuation of automatic extinction angle regulator 12B. Meanwhile, the operation characteristic curve of converter 1A is not changed from the case of FIG. 2, as indicated by portions (a), (b) and (c) in FIG. 3.
An intersecting point (A) of the two operation characteristic curves defines the operation point of the DC power transmission line. When the AC voltage of AC power line 6A coupled to converter 1A decreases under the above assumption, the regulation curve is shifted from portions (a) and (b) to portions (a') and (b') as indicated by the broken line in FIG. 3. Then, the operation characteristic curve of converter 1A becomes one being represented by the portions (a'), (b') and (c) in FIG. 3. From this, the operation point of the DC power transmission line is shifted from point (A) to a point (A') of FIG. 3. In this case, only a little DC current can flow and the power transfer from AC line 6A to AC line 6B is substantially disenabled. This is a serious problem.
In order to solve the above problem, the current margin is generally set at 5% to 10% of the rated current (IdA). When the current margin is set to fall within the range of 5% to 10%, the operation characteristic curve of converter 1B becomes one being represented by portions (d), (e), (f) and (g) in FIG. 2. Thus, even when the AC voltage of AC line 6A decreases and the operation characteristic curve of converter 1A is shifted from the portions (a), (b) and (c) to the portions (a'), (b') and (c) as indicated by the broken line in FIG. 2, the operation point of DC power transmission line is changed only from point (A) to a point (A') of FIG. 2. Thus, power transfer from AC line 6A to AC line 6B can be performed with only 5% to 10% decrease in the DC current. In this manner, to achieve a stable power transfer with a decreased voltage in the AC line of a forward converter (rectifier), in a prior art control system, a reverse converter (inverter) must always have a constant current characteristic with a current margin of about 5% to 10% with respect to the constant current (IdA) of the forward converter.
When the above prior art control system is applied to a DC power transmission line, a known technique which will be described below is applied to the transmission control circuits 26A and 26B in FIG. 1.
A DC power transmission line is generally provided with a control circuit for controlling the amount of power transfer between AC line 6A and AC line 6B. This control circuit produces a reference current value which corresponds to a current flowing through the power transmission line. The reference current value is supplied to the forward and reverse converters. For the sake of simplicity, the circuit for controlling the power transfer is called a reference current output circuit 27. An output signal E27 from reference current output circuit 27 is constantly supplied to transmission control circuit 26A and supplied, via a transmission line 19 of a communication system such as a microwave communication line, to transmission control circuit 26B.
As has been described before, when the currents at constant current characteristic portions of the forward and reverse converters are compared, the constant current characteristic portion (IdB in FIG. 2) of the reverse converter (inverter) is set to be smaller than that (IdA in FIG. 2) of the forward converter (rectifier) by a value corresponding to current margin value E25A or E25B. More specifically, the current margin value E25A or E25B is supplied to summing circuit 23A or 23B by proper selection of switches 24A and 24B, and the converter responsive to the selected current margin value operates as a reverse converter. Namely, when switch 24B is closed while switch 24A is opened and if reference current value E26A of converter 1A is equal to reference current value E26B of converter 1B, converter 1B operates as a reverse converter and has an operation characteristic curve of portions (d) to (g) as shown in FIG. 2. This characteristic curve is obtained by the function of the current margin and by the function of advanced control angle preference circuits 28A and 28B.
It is assumed that, for some reason, reference current value E26B of converter 1B becomes larger, by a value in excess of current margin E25B, than reference current value E26A of converter 1A. In this case, current IdB of the constant current characteristic portion of converter 1B becomes higher than current IdA of the constant current characteristic portion of converter 1A. This situation is equivalent to a state wherein, if reference current value E26B of converter 1B is apparently equal to reference current value E26A of converter 1A, the difference between current IdB and current IdA is supplied to converter 1A as a signal corresponding to the current margin. This means that the control mode is changed from one in which converter 1A operates as a forward converter while converter 1B operates as a reverse converter to one in which converter 1A operates as a reverse converter while converter 1B operates as a forward converter. Such a control mode change is called "power reversal". When a power reversal occurs, the power transfer from AC line 6A to AC line 6B is stopped, but that from AC line 6B to AC line 6A is carried out. This is a serious problem of the prior art converter control system.
In the above description, it was assumed that switch 24B of converter 1B is closed and that the reference current value E26B of converter 1B operating as a reverse converter would become larger than that E26A of converter 1A for some reason. One example of such a situation will be described below.
Now consideration will be given to a case where an output signal E27 from reference current output circuit 27 decreases. In this case, it is assumed that transmission control circuits 26A and 26B merely have a function to transfer or exchange given signals through microwave communication line 19. Under this assumption, reference current value E26A from transmission control circuit 26A is decreased with a decrease in output signal E27 from reference current output circuit 27 without substantial time delay, but reference current value E26B from transmission control circuit 26B is decreased with a certain time delay due to the presence of transmission line 19 of the communication system. In this case, regardless of the fact that normal signals are constantly transmitted via transmission line 19, reference current value E26B of converter 1B could temporarily become larger than reference current value E26A of converter 1A due to the above signal transmission time delay. This causes the problem of said power reversal.
In a prior art converter control system, in order to solve the above power reversal problem, transmission control circuits 26A and 26B must have not only the signal transfer function as described above but also have the following function. That is, when output signal E27 from reference current output circuit 27 decreases, the reference current value of a converter to be operated as a reverse converter is decreased first and then the reference current value of a converter to be operated as a forward converter is decreased. Conversely, when output signal E27 from reference current output circuit 27 increases, the reference current value of a converter to be operated as a forward converter is increased first and then the reference current value of a converter to be operated as a reverse converter is increased. Thus, transmission control circuits 26A and 26B transfer or exchange their given signals in a manner that the reference current value of a converter to be operated as a forward converter is always kept larger than that of a converter to be operated as a reverse converter. In other words, transmission control circuits 26A and 26B of the prior art system serve to transfer their signals so that a necessary current margin is always retained regardless of the increase or decrease of output signal E27 from reference current output circuit 27.
To solve the problem caused by a decrease in the AC line voltage at the forward converter (which has been already discussed with reference to FIG. 3), the reverse converter should have a specific constant current characteristic whose constant current value is determined by subtracting the current margin of 5% to 10% from the constant current value of the forward converter. For this purpose, the current margin value must be maintained at a certain value. However, when the communication system fails to operate normally due to disturbance etc., or when communication via the microwave communication line is interrupted, the reference current value determining the amount of power transfer cannot be properly changed. This would cause a disadvantageous situation that, even if output signal E27 from reference current output circuit 27 changes, transmission line 19 of the communication system cannot transmit the corresponding signal change properly. In other words, the reference current value of a converter to be operated as a reverse converter could become larger than that of a converter to be operated as a forward converter, resulting in the occurrance of said power reversal.